A current challenge for optical communications is to expand the capacities of optical backbone network to cope with the possible future explosive expansion of information communications traffic. Various approaches are being taken to the challenge. One of the approaches is to carry out research and development regarding an improvement in optical bandwidth use efficiency.
In optical networks, optical bandwidths are used in accordance with the Dense Wavelength Division Multiplexing (DWDM) system standardized by the Telecommunication Standardization sector of the International Telecommunication Union (ITU-T). In the DWDM system, the entire available optical bandwidth is divided into narrow segments by a grid with constant width, called a wavelength grid, and optical signals in one wavelength channel are allocated within a grid spacing (ITU-T recommendation G.694.1).
For example, in an optical network with a grid spacing of 50 GHz, an optical bandwidth occupied by any optical signal needs to be less than or equal to 50 GHz. The optical bandwidth occupied by an optical signal with a transmission rate of 40 Gbps (Gigabits per second) is approximately 50 GHz taking into account a guard band for avoiding interference with adjacent channels. Accordingly, if the transmission rates of all optical signals used in an optical network with a grid spacing of 50 GHz are 40 Gbps, the optical bands available in the optical network can be used without any space. However, if there is a 10-Gbps optical signal with an optical bandwidth of approximately 15 GHz among the optical signals, an optical band of 35 GHz in the 50-GHz grid remains unused and cannot be allocated to another optical signal.
Further, focusing on transmission distances, there is the following wasteful allocation of optical bands even if the transmission rates of all optical signals are 40 Gbps in an optical network with a grid spacing of 50 GHz as the above-mentioned example. If all of the optical bands with 50 GHz are occupied, the distance over which an optical signal can be transmitted without optical termination is approximately 500 km. Accordingly, if optical path length is shorter than 500 km, an optical signal can be transmitted with an optical bandwidth narrower than 50 GHz that is actually allocated. For example, an optical path is considered that has a transmission rate of 40 Gbps and an optical path length of 250 km. Assuming that the minimum required optical bandwidth is 25 GHz, an optical bandwidth of 25 GHz is allocated excessively in an optical network with a grid spacing of 50 GHz and may not be allocated to another optical signal.
As a technology to solve these problems, an elastic optical network technique in which a flexible frequency grid is used has been proposed (for example, see PTL 1). The flexible frequency grid is a technique that supports wavelength division multiplexing that is obtained by further sophisticating DWDM and is standardized in ITU-T (ITU-T recommendation G.694.1). In the flexible frequency grid, the grid spacing is fractionized more finely than that of DWDM frequency grid. The grid spacing to be allocated to an optical path is variable and can individually be set for each optical path. This allows a minimum required optical bandwidth to be allocated to an optical path in an elastic optical network depending on the optical path length and traffic amount, which can improve the use efficiency of the optical bands.